The abbreviations in this application for the chemical elements are those used for the Periodic Table.
The use of x-rays to take pictures of the human body is almost 100 years old. The standard x-ray tube (a point source) and film (a spatially distributed detector) are commonly used throughout the medical world. Often, the film, which has a very low sensitivity or efficiency to x-ray photons, is employed together with a fluorescing screen which is placed directly in front of the film. Using this technique, reasonably high efficiencies of x-ray photon absorption can be achieved. The spatial resolution obtainable depends upon the film used; the very best film provides resolutions of the order of 18 line pairs per millimeter, about 50 microns in space. There is no energy resolution possible in the standard system, but this is unimportant since most x-ray sources are very broad in energy.
The human body absorbs most x-ray photons below about 30 keV. Thus, most standard x-ray machines use a tungsten (W) or other heavy metal target and an incident electron beam of 60 or more keV. Radiology is typically conducted at energies up to 90 or so keV.
The use of the point x-ray source and spatially distributed film detector has been adopted for mammographic uses. Soft tissue radiology, such as mammography, uses a much lower energy system. Here a molybdenum (hereinafter referred to as "Mo") target and an electron beam of about 25 keV is used. The amount of tissue to be penetrated is not great, and there is no bone. Small calcifications represent one of the many signs that radiologists seek in their search for possible breast cancers.
Mo emits a spectrum of x-rays up to the maximum energy of the electron beam (.about.25 keV) but with peaks at about 17 and 19 keV due to its atomic structure. A typical spectrum is shown in FIG. 1 (the Mo spectrum as shown in Medical Imaging Physics, 3rd ed., Hend, W. R. & Ritenour, R., p. 131). The Mo target is followed by a very thin foil of Mo (generally about 30 micrometers). This foil emphasizes the two lines produced, at 17 and 19 KeV, by reducing the flat background radiation.
The use of so-called reverse geometry x-rays has also been noted. A reverse geometry distributed source of x-radiation with a point detector has been developed by DigiRay Co., San Ramon, Calif. (which is the assignee of U.S. Pat. Nos. 3,949,229; 4,259,582; 4,465,540; and 5,267,296, all to R. D. Albert). In these systems, a point detector, usually an inorganic crystal such as NaI, is used in conjunction with a scanned x-ray source. The x-ray source consists of an electron beam striking a metal target, but the beam is scanned across the target in a fashion similar to the raster scan of a television tube, i.e. a raster-scanned x-ray beam is produced.
The idea of making a digital radiographic system to replace the presently-used analog film recording has a history of over 20 years. The advantages of digital radiography are numerous and have been discussed at length in the literature. Generally, digital detectors have been used to replace film directly. For many reasons, the replacement of film has never taken hold and digital systems continue to be experimental in nature.
Microstrip detectors (also called "crossed-strip microstrip detector" or "crossed-strip detector") for charged particles for two-dimensional imaging have been in use in high-energy physics experiments for several years to detect ionizing particles. They have been used with charged particles which penetrate the 300 .mu.m silicon (Si) detector. These detectors can have spatial resolution down to less than 20 .mu.m. By taking two of these detectors at right angles to one another, it is easily possible to get both x and y knowledge of the particle's position. Two-dimensional detectors {Krummenacher, F. et al., Nucl. Instruments& Methods Phy. Res., A288: 176-179 (1990)} are known in the art and are used to produce two-dimensional readout {see e.g., Campbell, M. et al., Nuclear Inst. & Methods in Phy. Res., A290:149-157 (1990)}. The information from the detector elements can be in the form of analog signals generated by individual particles or photons, or alternatively, it can be the total amount of charge integrated in an element during a time interval. In both cases, the signals could be processed through analog-to-digital conversion or through a discriminator (threshold comparison or 1-bit analog-to-digital converter (ADC)) {Heijne, E. H. M., et al., Nuclear Inst. & Methods in Phy. Res., A275:467-471 (1989)}. The semiconductor detector thus provides a direct link to digital information processing.
B. Alfano et al., produced two-dimensional x-ray images using a point source x-ray generator and a double-sided microstrip silicon (hereinafter referred to as "Si") detector. {Alfano, et al., Phy. Med. Biol., 37(5):1167-1170 (1992).} The measurements were performed with photons emitted from two different sources, namely .sup.109 Cd and .sup.241 Am. Alfano, et al. used a silicon crystal 300 .mu.m thick, 1.4.times.1.4 cm.sup.2 surface area with microstrips deposited on each side to give two orthogonal coordinates in the plane normal to the incoming photon. The electrodes, 12 .mu.m wide, were deposited in arrays with 25 .mu.m spacing on the junction (J) side and with 50 .mu.m spacing on the ohmic (.OMEGA.) side. The read-out pitch was 100 .mu.um for both sides. A limited number of channels were equipped with standard preamplifier+amplifier "front-end" electronics. The signal of each channel was sent both to an analog-to-digital converter (ADC). Images were obtained by exposing to the 60 keV photons, from the .sup.109 Cd and .sup.241 Am sources, the double-sided microstrip silicon detector with tantalum wires, i.e. high contrast objects as phantoms.